PROJECT SUMMARY/ABSTRACT Aging is accompanied by a decreased tolerance to physiological stress, which promotes increased susceptibility to inflammatory illnesses. Critical illnesses such as sepsis, disportionately effect people over the age of 65, leading to increased morbidity and mortality in older adults. Thrombosis is a common complication from sepsis, contributing to organ failure and death. The significance of this devastating dysregulated host response is demonstrated by data showing half of all hospital intensive care admissions are from individuals over the age of 65 and may be attributed to infection. Emerging evidence supports the concept that dysregulated platelet functions mediate the injurious host response during inflammation. Nevertheless, the molecular mechanisms and functional consequences of dysregulated platelet functions during aging and inflammation remain incompletely understood. Our proposal, entitled ?Interferon-Induced Transmembrane Protein 3 (IFITM3) Regulates Thrombosis During Inflammation in Aging? will identify new pathways by which inflammatory agonists, including interferons (IFNs), regulate gene expression in platelets and their parent cell, the megakaryocyte (MK), in aging. Our preliminary studies have identified that the expression of IFITM3 is robustly induced in human platelets during sepsis, a systemic inflammatory illness. Interestingly, MKs and platelets from aged human and mice express more IFITM3 after IFN stimulation compared to younger controls. Our data suggest that IFITM3 promotes fibrinogen endocytosis in MKs and platelets, leading to platelet hyperreactivity and thrombosis. Our findings also suggest that in aging and during inflammatory stress, the mammalian target of rapamycin pathway is activated, triggering IFITM3 synthesis and thrombosis. The expression and function of IFITM3 in MKs and platelets and its regulation by mTOR is a pathway not previously examined. In this proposal, we will couple studies in older and younger septic patients with in vitro and in vivo murine models using aged mice. These complementary human and murine studies will allow us to establish clinical relevance, while also dissecting the mechanisms by which IFITM3 governs MK and platelet function during inflammation. These studies are translational and innovative as IFITM3 regulation of endocytosis, a process critical for cellular function, has not previously been studied in MKs, platelets, or ? for that matter - any primary human cells. They will also determine for the first time whether aging alters the effect of inflammatory agonists on transcriptional and translational events in MKs and platelets. This work will test an important functional hypothesis and clarify pathophysiologic mechanisms of thrombosis aging during inflammation. This proposal has translational potential for older patients with sepsis, and also will uncover new pathways linking thrombosis and inflammation in aging.